Optical modulator and optical transmitter

ABSTRACT

An optical modulator includes a package that accommodates therein a first substrate and a second substrate different from the first substrate, and outside the package, a flexible circuit board. The first substrate has plural optical modulating units disposed thereon in parallel and each including a Mach-Zehnder optical waveguide. Plural first signal line paths corresponding to the optical modulating units are disposed on the second substrate. Plural second signal line paths corresponding to the optical modulating units are disposed on the flexible circuit board. Electrical lengths of the second signal line paths are different from one another. Electrical lengths of signal paths that span from input ends of the second signal line paths corresponding to the optical modulating units to base points on signal electrodes, via the first signal line paths, are equal to one another.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2012-239064, filed on Oct. 30,2012, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to an optical modulator andan optical transmitter.

BACKGROUND

A conventional optical modulator includes a package accommodating asubstrate for optical modulation including an optical waveguide andplural signal electrodes interacting with the optical waveguide; and awiring substrate including wires connected to the signal terminals.Among such optical modulators is an optical modulator that has wiringwith numerous turns on the wiring substrate such that the electricallength of the wires differ from one another, whereby the opticalmodulator adjusts the phase differences among high-frequency electricalsignals input into the signal electrodes (see, e.g., DomesticRepublication of PCT International Publication for Patent Application,Publication No. 2010/021193). Another optical modulator uses a flexiblewiring board as a wiring substrate (see, e.g., Japanese Laid-Open PatentPublication No. 2011-138049).

However, in such conventional optical modulators, the wires are disposedto have numerous turns on the wiring substrate and therefore, the sizeof the wiring substrate increases. The wiring substrate is disposedinside the package and therefore, a problem arises in that the size ofthe package increases by an amount corresponding to the increase in thesize of the wiring substrate.

SUMMARY

According to an aspect of an embodiment, an optical modulator includes afirst substrate that has a Mach-Zehnder optical waveguide that is formedon a substrate having an electro-optical effect; a signal electrode anda ground electrode that are disposed along a pair of branchingwaveguides positioned between an optical branching unit and an opticalcoupling unit of the Mach-Zehnder optical waveguide; plural opticalmodulating units that are disposed in parallel, and respectivelymodulate a light beam propagating in the Mach-Zehnder optical waveguideby applying to the signal electrode to be a travelling wave electrode,an electrical signal that corresponds to modulation data; and an outputoptical coupling unit that couples modulated light beams output from theoptical modulating units. The optical modulator further has a secondsubstrate that is disposed separately from the first substrate and hasplural first signal line paths corresponding to the optical modulatingunits; a package that accommodates therein the first substrate and thesecond substrate; and a flexible circuit board that is disposed outsidethe package and has plural second signal line paths corresponding to theoptical modulating units. Input ends of input electrodes of the opticalmodulating units on the first substrate are each supplied the electricalsignal and are disposed side by side on one side face of the firstsubstrate. When base points are independently set such that each opticalpath length from the output light beam wave coupling unit in a branchingwaveguide disposed along the signal electrode of each pair of branchingwaveguides of the optical modulating units is equivalent, eachelectrical length from an input end of each signal electrode to eachrespective base point differs. Input ends of the first signal line pathson the second substrate are respectively supplied the electrical signalsrespectively corresponding to the optical modulating units and aredisposed side by side on one side face of the second substrate. Outputends electrically connected to input ends of the signal electrodes ofthe corresponding optical modulating units on the first substrate aredisposed side by side on another side face of the second substrate.Input ends of the second signal line paths on the flexible circuit boardare respectively supplied the electrical signals respectivelycorresponding to the optical modulating units and are disposed side byside on one side face of the flexible circuit board, and output endselectrically connected to input ends of the corresponding first signalline paths on the second substrate are disposed side by side on anotherside face of the flexible circuit board. Electrical lengths from theinput ends of the second signal line paths to output ends thereofrespectively differ. Electrical lengths of signal paths respectivelycorresponding to the optical modulating units are equivalent, the signalpaths include signal paths that are from the input ends of the secondsignal line paths and pass through input ends of the first signal linepaths connected to the output ends of the second signal line paths, tothe base points on the signal electrodes, and signal paths that are fromthe input ends of the first signal line paths and pass through the inputends of the signal electrodes connected to output ends of the firstsignal line paths, to the base points on the signal electrodes.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram of a first example of an opticalmodulator according an embodiment;

FIG. 2 is an explanatory diagram of another example of the opticalmodulator;

FIG. 3 is an explanatory diagram of a second example of the opticalmodulator according to the embodiment;

FIG. 4 is an explanatory diagram of a third example of the opticalmodulator according to the embodiment;

FIG. 5 is an explanatory diagram of a fourth example of the opticalmodulator according to the embodiment;

FIG. 6 is an explanatory diagram of a fifth example of the opticalmodulator according to the embodiment;

FIG. 7 is an explanatory diagram of a sixth example of the opticalmodulator according to the embodiment;

FIG. 8 is an explanatory diagram of a seventh example of the opticalmodulator according the embodiment;

FIG. 9 is an explanatory diagram of an eighth example of the opticalmodulator according to the embodiment; and

FIG. 10 is an explanatory diagram of an example of an opticaltransmitter according to the embodiment.

DESCRIPTION OF EMBODIMENTS

A preferred embodiment of an optical modulator and an opticaltransmitter will be described in detail with reference to theaccompanying drawings. In the description of each of the followingexamples, identical components are given identical reference numeralsand redundant description is omitted.

FIG. 1 is an explanatory diagram of a first example of an opticalmodulator according the embodiment. As depicted in FIG. 1, an opticalmodulator 1 includes a package 2 that accommodates therein a firstsubstrate 3 and a second substrate 4 different from the first substrate3; and a flexible circuit board 5 outside the package. The firstsubstrate 3 is a substrate for an optical modulator chip.

The first substrate 3 may be a z-cut crystal substrate having anelectro-optical effect such as, for example, LiNbO₃ (hereinafter, simplyreferred to as “LN”) or LiTaO₂. An optical waveguide device using suchan electro-optical crystal may be formed by forming an optical waveguideof a metal film such as Ti on a portion of the crystal substrate andcausing the film to thermally diffuse, or by exchanging protons inbenzoic acid after patterning; and by disposing electrodes in thevicinity of the optical waveguide.

Plural optical modulating units are disposed in parallel on the firstsubstrate 3. In the example depicted in FIG. 1, two optical modulatingunits 6 and 7 are disposed in parallel, where the first opticalmodulating unit 6 is located in an upper position in FIG. 1 and thesecond optical modulating unit 7 is located in a lower position inFIG. 1. Although the number of optical modulating units disposed inparallel is not limited two, the description will be made for theembodiment taking a case where the number of optical modulating units istwo, as an example. The first and the second optical modulating units 6and 7 each include a Mach-Zehnder optical waveguide formed on thesubstrate that has the electro-optical effect.

In the first optical modulating unit 6, the Mach-Zehnder opticalwaveguide includes a first input waveguide 8, a first optical branchingunit 9, a first and a second branching waveguides 10 and 11, a firstoptical coupling unit 12, and a first output waveguide 13. The first andthe second branching waveguides 10 and 11 are disposed in parallelbetween the first optical branching unit 9 and the first opticalcoupling unit 12.

In the first optical modulating unit 6, a first signal electrode 14 isdisposed along the first branching waveguide 10 and ground electrodes 15and 16 are disposed along the second branching waveguide 11. The groundelectrodes 15 and 16 and the first signal electrode 14 form a coplanarelectrode. When a Z-cut substrate is used, the first signal electrode 14is disposed immediately above the first branching waveguide 10 and theground electrode 16 is disposed immediately above the second branchingwaveguide 11. Thereby, variation of the refraction index can be usedthat is caused by the electric field in the Z-direction.

A buffer layer having a thickness of, for example, about 0.2 to 2 μmformed using SiO₂, etc., may be disposed between the electro-opticalcrystal, and the first signal electrode 14 and the ground electrode 16.Thereby, light beams propagating in the first and the second branchingwaveguides 10 and 11 are prevented from being absorbed respectively bythe first signal electrode 14 and the ground electrode 16.

In the second optical modulating unit 7, the Mach-Zehnder opticalwaveguide includes a second input waveguide 18, a second opticalbranching unit 19, a third and a fourth branching waveguides 20 and 21,a second optical coupling unit 22, and a second output waveguide 23. Thethird and the fourth branching waveguides 20 and 21 are disposed inparallel between the second optical branching unit 19 and the secondoptical coupling unit 22.

In the second optical modulating unit 7, a second signal electrode 24 isdisposed along the third branching waveguide 20 and ground electrodes 25and 16 are disposed along the fourth branching waveguide 21. The groundelectrodes 25 and 16 and the second signal electrode 24 form a coplanarelectrode. When the Z-cut substrate is used, the second signal electrode24 is disposed immediately above the third branching waveguide 20 andthe ground electrode 25 is disposed immediately above the fourthbranching waveguide 21. Thereby, variation of the refraction index canbe used that is caused by the electric field in the Z-direction.

A buffer layer having a thickness of, for example, about 0.2 to 2 μmformed using SiO₂, etc., may be disposed between the electro-opticalcrystal, and the second signal electrode 24 and the ground electrode 25.Thereby, light beams propagating in the third and the fourth branchingwaveguides 20 and 21 are prevented from being absorbed respectively bythe second signal electrode 24 and the ground electrode 25.

An input end 31 supplied with an electrical signal and corresponding tothe first optical modulating unit 6, of the first signal electrode 14,and an input end 32 supplied with the electrical signal andcorresponding to the second optical modulating unit 7, of the secondsignal electrode 24 are disposed side by side on one side face of thefirst substrate 3. In the example depicted in FIG. 1, the input ends 31and 32 of the first and the second signal electrodes 14 and 24 aredisposed side by side on a lower side face of the first substrate 3.

Ends 33 and 34 of the first and the second signal electrodes 14 and 24are disposed side by side in an end portion 35 on another side face ofthe first substrate 3. In the example depicted in FIG. 1, the ends 33and 34 of the first and the second signal electrodes 14 and 24 aredisposed side by side on the upper side face of the first substrate 3 inFIG. 1.

In the end portion 35, the end 33 of the first signal electrode 14 isconnected to an end of the ground electrode 16 through a resistor notdepicted. Thereby, the first signal electrode 14 is a travelling waveelectrode. When a first electrical signal of a microwave correspondingto modulation data is applied to the input end 31 of the first signalelectrode 14, the refraction indexes of the first and the secondbranching waveguides 10 and 11 are respectively varied by +Δn₁ and −Δn₂by the generated electric field. Thereby, the phase difference is variedbetween the first and the second branching waveguides 10 and 11, and asignal light beam output from the first output waveguide 13 isintensity-modulated by Mach-Zehnder interference.

In the end portion 35, the end 34 of the second signal electrode 24 isconnected to an end of the ground electrode 25 through a resistor notdepicted. Thereby, the second signal electrode 24 is a travelling waveelectrode. When a second electrical signal of a microwave correspondingto the modulation data is applied to the input end 32 of the secondsignal electrode 24, the refraction indexes of the third and the fourthbranching waveguides 20 and 21 are respectively varied by +Δn₃ and −Δn₄by the generated electric field. Thereby, the phase difference is variedbetween the third and the fourth branching waveguides 20 and 21, and asignal light beam output from the second output waveguide 23 isintensity-modulated by Mach-Zehnder interference.

Varying the cross-sectional shapes of the first and the second signalelectrodes 14 and 24 can control the effective refraction indexes of themicrowaves. Thereby, the speeds of the light beam and the microwave canbe matched with each other and a high-speed optical responsiveness canbe acquired. The optical modulator 1 depicted in FIG. 1 can produce amulti-level modulation signal based on the fact that the first and thesecond electrical signals are different from each other.

An input optical branching unit 26 and an output optical coupling unit27 are formed on the first substrate 3. An optical input end of theinput optical branching unit 26 may be connected to, for example, anoptical fiber 42 through a connector 41. One optical output end of theinput optical branching unit 26 is connected to the first inputwaveguide 8. The other optical output end of the input optical branchingunit 26 is connected to the second input waveguide 18. The input opticalbranching unit 26 branches, for example, an input light beam input fromthe optical fiber 42 through the connector 41 into two light beams atintensity ratio of, for example, 1:1 and outputs the two light beams tothe first and the second input waveguides 8 and 18.

One optical input end of the output optical coupling unit 27 isconnected to the first output waveguide 13. The other optical input endof the output optical coupling unit 27 is connected to the second outputwaveguide 23. The optical output end of the output optical coupling unit27 may be connected to, for example, an optical fiber 44 through aconnector 43. The output optical coupling unit 27 may couple themodulated light beams output from the first and the second outputwaveguides 13 and 23, and may output the coupled light beam to theoptical fiber 44 through the connector 43.

For each of the first and the third branching waveguides 10 and 20, abase point is independently set such that the optical path length fromthe output optical coupling unit 27 to the base point is equivalent forthe first and the third branching waveguides 10 and 20. In thedescription below, the base point in the first branching waveguide 10may be denoted by “P_(C1)” and the base point in the third branchingwaveguide 20 may be denoted by “P_(C2)”. Such electrical lengths aredifferent from each other as those respectively from the input ends 31and 32 of the first and the second signal electrodes 14 and 24 to thebase points P_(C1) and P_(C2) in the first and the third branchingwaveguides 10 and 20.

The second substrate 4 may be, for example, a ceramic substrate, andhave plural first signal line paths 51 and 52 disposed thereon thatrespectively correspond to the optical modulating units 6 and 7. In thedescription below, the first signal line path 51 corresponding to thefirst optical modulating unit 6 may be referred to as “first-1 signalline path 51” and the first signal line path 52 corresponding to thesecond optical modulating unit 7 may be referred to as “first-2 signalline path 52”.

An input end 53 of the first-1 signal line path 51 is supplied theelectrical signal and corresponds to the first optical modulating unit6, and an input end 54 of the first-2 signal line path 52 is suppliedthe electrical signal and corresponds to the second optical modulatingunit 7. The input ends 53 and 54 are disposed side by side on one sideface of the second substrate 4. In the example depicted in FIG. 1, theinput ends 53 and 54 of the first-1 and the first-2 signal line paths 51and 52 are disposed side by side on the lower side face of the secondsubstrate 4 in FIG. 1.

An output end 55 of the first-1 signal line path 51 and an output end 56of the first-2 signal line path 52 are disposed side by side on theother side face of the second substrate 4. In the example depicted inFIG. 1, the output ends 55 and 56 of the first-1 and the first-2 signalline paths 51 and 52 are disposed side by side on the upper side face ofthe second substrate 4 in FIG. 1.

The output end 55 of the first-1 signal line path 51 may electrically beconnected to the input end 31 of the first signal electrode 14 by, forexample, wire bonding. The output end 56 of the first-2 signal line path52 may electrically be connected to the input end 32 of the secondsignal electrode 24 by, for example, wire bonding. An electrode pad notdepicted may be disposed for each of the output end 55 of the first-1signal line path 51, the output end 56 of the first-2 signal line path52, the input end 31 of the first signal electrode 14, and the input end32 of the second signal electrode 24, and a wire may be bonded to theelectrode pad.

The flexible circuit board 5 has plural second signal line paths 61 and62 disposed thereon that respectively correspond to the opticalmodulating units 6 and 7. In the description below, the second signalline path 61 corresponding to the first optical modulating unit 6 may bereferred to as “second_1 signal line path 61” and the second signal linepath 62 corresponding to the second optical modulating unit 7 may bereferred to as “second_2 signal line path 62”.

An input end 63 of the second_1 signal line path 61 is supplied theelectrical signal and corresponds to the first optical modulating unit6, and an input end 64 of the second_2 signal line path 62 is suppliedthe electrical signal and corresponds to the second optical modulatingunit 7. The input ends 63 and 64 are disposed side by side on one sideface of the flexible circuit board 5. In the example depicted in FIG. 1,the input ends 63 and 64 of the second_1 and the second_2 signal linepaths 61 and 62 are disposed side by side on the lower side face of theflexible circuit board 5 in FIG. 1.

An output end 65 of the second_1 signal line path 61 and an output end66 of the second_2 signal line path 62 are disposed side by side on theother side face of the flexible circuit board 5. In the example depictedin FIG. 1, the output end 65 of the second_1 signal line path 61 and theoutput end 66 of the second_2 signal line path 62 are disposed side byside on the upper side face of the flexible circuit board 5 in FIG. 1.

The output end 65 of the second_1 signal line path 61 may electricallybe connected to, for example, the input end 53 of the first-1 signalline path 51 through an electric connector not depicted disposed on theside face of the package 2. The output end 66 of the second_2 signalline path 62 may electrically be connected to, for example, the inputend 54 of the first-2 signal line path 52 through an electric connectornot depicted disposed on the side face of the package 2.

The input portions for the electrical signals to the optical modulator 1are disposed side by side on the one side of the package 2 as above andthereby, the optical modulator 1 can easily be mounted on, for example,a printed circuit board 71. The area to have the optical modulator 1mounted therein can be reduced. For example, the printed circuit board71 and the input portions of the electrical signals to the opticalmodulator 1 are connected to each other using the flexible circuit board5 and, thereby, the area to have the optical modulator 1 mounted thereincan be reduced.

Electrical lengths are different from each other such as that from theinput end 63 of the second_1 signal line path 61 to the output end 65 ofthe second_1 signal line path 61 and that from the input end 64 of thesecond_2 signal line path 62 to the output end 66 of the second_2 signalline path 62. For example, in the example depicted in FIG. 1, thesecond_1 signal line path 61 extends in a straight line in a directionthat crosses at a right angle the longitudinal direction of the firstsubstrate 3, that is, the widthwise direction of the first substrate 3.

On the other hand, the second_2 signal line path 62: extends from theinput end 64 in the widthwise direction of the first substrate 3; curvesmidway into the longitudinal direction of the first substrate 3; againcurves into the widthwise direction of the first substrate 3; againcurves into a reverse direction of the longitudinal direction of thefirst substrate 3; again curves into the widthwise direction of thefirst substrate 3; and reaches the output end 66. The electrical lengthof the second_2 signal line path 62 is longer than the electrical lengthof the second_1 signal line path 61 by the length that corresponds tothe length for the second_2 signal line path 62 to go into thelongitudinal direction of the first substrate 3 and return therefrommaking the four curves.

Electrical lengths are equal to each other such as that of the signalpath from the input end 63 of the second_1 signal line path 61 to thebase point P_(C1) of the first signal electrode 14 and that of thesignal path from the input end 64 of the second_2 signal line path 62 tothe base point P_(C2) of the second signal electrode 24. Thereby, whenthe electrical signal corresponding to the modulation data issimultaneously applied to each of the input ends 63 and 64 of thesecond_1 and second_2 signal line paths 61 and 62, the electricalsignals simultaneously reach the interaction portion of the firstbranching waveguide 10 and the first signal electrode 14, and theinteraction portion of the third branching waveguide 20 and the secondsignal electrode 24.

Therefore, timings of such modulated light beams match with each otheras that output from the first output waveguide 13 and that output fromthe second output waveguide 23. “Zero skew” is realized between thefirst and the second optical modulating units 6 and 7.

The loss per unit length of the flexible circuit board 5 will be denotedby “α_(f)”, the loss per unit length of the second substrate 4 will bedenoted by “α_(c)”, and the loss per unit length of the first substrate3 will be denoted by “α_(m)”. As to the second signal line paths 61 and62 on the flexible circuit board 5, the length of the second_1 signalline path 61 will be denoted by “L_(f1)” and the length of the second_2signal line path 62 will be denoted by “L_(f2)”. As to the first signalline paths 51 and 52 on the second substrate 4, the length of thefirst-1 signal line path 51 will be denoted by “L_(c1)” and the lengthof the first-2 signal line path 52 will be denoted by “L_(c2)”. As tothe first signal electrode 14 on the first substrate 3, the length fromthe input end 31 to the base point P_(C1) will be denoted by “L_(m1)”and, as to the second signal electrode 24 on the first substrate 3, thelength from the input end 32 to the base point P_(C2) will be denoted by“L_(m2)”.

Such structures only have to be designed such that the followingequation holds, as the signal electrodes 14 and 24, the first signalline paths 51 and 52, and the second signal line paths 61 and 62.Thereby, any difference in the band can be prevented from beinggenerated between the first and the second optical modulating units 6and 7. The bands of the first and the second optical modulating units 6and 7 can be equalized.α_(f)×L_(f1)+α_(c)×L_(c1)+α_(m)×L_(m1)=α_(f)×L_(f2)+α_(c)×L_(c2)+α_(m)×L_(m2)

The optical modulator 1 may be mounted on, for example, the printedcircuit board 71. The input end 63 of the second_1 signal line path 61may electrically be connected, for example, by soldering to a firstsignal wire 72 that is formed on the printed circuit board 71. The inputend 64 of the second_2 signal line path 62 may electrically beconnected, for example, by soldering to a second signal wire 73 that isformed on the printed circuit board 71. The first and the second signalwires 72 and 73 may be connected to a driver not depicted that drivesthe optical modulator 1. The driver may be mounted on, for example, theprinted circuit board 71.

FIG. 2 is an explanatory diagram of another example of the opticalmodulator. In an optical modulator depicted in FIG. 2, the first-2signal line path 52 is formed in a shape to have many curves on thesecond substrate 4 in the package 2 for the electrical length of thefirst-2 signal line path 52 to be set to be longer than that of thefirst-1 signal line path 51 and, thereby, the “zero skew” is realizedbetween the optical modulating units 6 and 7. The electrical lengths ofthe second_1 and the second_2 signal line paths 61 and 62 are equal toeach other on the flexible circuit board 5.

According to the optical modulator 1 depicted in FIG. 1, the electricallengths are adjusted on the flexible circuit board 5 outside the package2 and, thereby, the length in the widthwise direction of the firstsubstrate 3 of the second substrate 4 can be shortened compared to theconfiguration to adjust the electrical lengths on the second substrate 4in the package 2 as in the example depicted in FIG. 2. Therefore, thesize of the package 2 can be reduced.

FIG. 3 is an explanatory diagram of a second example of the opticalmodulator according to the embodiment. In an optical modulator 1depicted in FIG. 3, an interval D_(f) between the input end 63 of thesecond_1 signal line path 61 and the input end 64 of the second_2 signalline path 62 is different from an interval D_(c) between the input end53 of the first-1 signal line path 51 and the input end 54 of thefirst-2 signal line path 52 in the optical modulator 1 depicted in FIG.1.

In the example depicted in FIG. 3, the interval D_(f) between the inputend 63 of the second_1 signal line path 61 and the input end 64 of thesecond_2 signal line path 62 is longer than the interval D_(c) betweenthe input end 53 of the first-1 signal line path 51 and the input end 54of the first-2 signal line path 52. The other configurations in thesecond example are same as those of the optical modulator 1 depicted inFIG. 1 and therefore, the same configurations are given the samereference numerals and will not again be described.

According to the optical modulator 1 depicted in FIG. 3, for example,even when an interval between the signal wires 72 and 73 formed on theprinted circuit board 71 is longer than an interval between the inputends 31 and 32 of the signal electrodes 14 and 24 on the first substrate3, the pitch can be converted on the flexible circuit board 5. Theconversion of the pitch on the flexible circuit board 5 enablesreduction of the length in the widthwise direction of the firstsubstrate 3 of the second substrate 4 compared to the case where thepitch is converted on the second substrate 4. Therefore, the size of thepackage 2 can be reduced.

FIG. 4 is an explanatory diagram of a third example of the opticalmodulator according to the embodiment. In an optical modulator 1depicted in FIG. 4, the interval D_(f) between the input end 63 of thesecond_1 signal line path 61 and the input end 64 of the second_2 signalline path 62 is longer than the interval D_(c) between the input end 53of the first-1 signal line path 51 and the input end 54 of the first-2signal line path 52 in the optical modulator 1 depicted in FIG. 1.

In the optical modulator 1 depicted in FIG. 4, the interval D_(c)between the input end 53 of the first-1 signal line path 51 and theinput end 54 of the first-2 signal line path 52 is longer than theinterval D_(m) between the input end 31 of the first signal electrode 14and the input end 32 of the second signal electrode 24 in the opticalmodulator 1 depicted in FIG. 1. The other configurations in the thirdexample are same as those of the optical modulator 1 depicted in FIG. 1and therefore, the same configurations are given the same referencenumerals and will not again be described.

According to the optical modulator 1 depicted in FIG. 4, for example,even when the interval between the signal wires 72 and 73 formed on theprinted circuit board 71 is longer than the interval between the inputends 31 and 32 of the signal electrodes 14 and 24 on the first substrate3, the pitch can be converted on each of the flexible circuit board 5and the second substrate 4. The conversion of the pitch on each of theflexible circuit board 5 and the second substrate 4 enables reduction ofthe length in the widthwise direction of the first substrate 3 of thesecond substrate 4 compared to the case where the conversion of thepitch is executed only on the second substrate 4. Therefore, the size ofthe package 2 can be reduced.

In the case where the propagation loss on the flexible circuit board 5is significant, the propagation loss is increased when the pitch isconverted only on the flexible circuit board 5 and therefore, aninconvenience may occur that the band degradation is significant. Theconversion of the pitch on each of the flexible circuit board 5 and thesecond substrate 4 as in the optical modulator 1 depicted in FIG. 4enables suppression of the propagation loss on the flexible circuitboard 5 and suppression of the degradation of the bands compared to thecase where the conversion of the pitch is executed only on the flexiblecircuit board 5.

FIG. 5 is an explanatory diagram of a fourth example of the opticalmodulator according to the embodiment. In an optical modulator 1depicted in FIG. 5, a central line C_(f) between the input end 63 of thesecond_1 signal line path 61 and the input end 64 of the second_2 signalline path 62 deviates from a central line C_(c) between the input end 53of the first-1 signal line path 51 and the input end 54 of the first-2signal line path 52 in the optical modulator 1 depicted in FIG. 1.

The second_2 signal line path 62 may be disposed obliquely to thelongitudinal direction of the first substrate 3. The second_1 signalline path 61 may be disposed obliquely to the longitudinal direction ofthe first substrate 3 matched with the interval between the input end 63of the second_1 signal line path 61 and the input end 64 of the second_2signal line path 62.

The interval between the input end 63 of the second_1 signal line path61 and the input end 64 of the second_2 signal line path 62 may belonger than the interval between the input end 53 of the first-1 signalline path 51 and the input end 54 of the first-2 signal line path 52 asin the optical modulator 1 depicted in FIG. 3. The interval between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 may be longer than the interval betweenthe input end 31 of the first signal electrode 14 and the input end 32of the second signal electrode 24 as in the optical modulator 1 depictedin FIG. 4. Other components in the fourth example are identical to thoseof the optical modulator 1 depicted in FIG. 1 and therefore, are giventhe same reference numerals used in FIG. 1 and will not again bedescribed.

According to the optical modulator 1 depicted in FIG. 5, the electricallength of the second_2 signal line path 62 can be set to be longer thanthe electrical length of the second_1 signal line path 61 withoutcausing the second_2 signal line path 62 to curve for many times.Therefore, the size of the flexible circuit board 5 can be reduced. Theoblique disposition of the second_2 signal line path 62 enablesreduction of the length of the second_2 signal line path 62. Therefore,the band of the second optical modulating unit 7 can be widened.

FIG. 6 is an explanatory diagram of a fifth example of the opticalmodulator according to the embodiment. In an optical modulator 1depicted in FIG. 6, the central line C_(f) between the input end 63 ofthe second_1 signal line path 61 and the input end 64 of the second_2signal line path 62 deviates from the central line C_(c) between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 in the optical modulator 1 depicted inFIG. 1.

The second_2 signal line path 62 and the first-2 signal line path 52 aredisposed each in a straight line and obliquely to the longitudinaldirection of the first substrate 3. The second_1 signal line path 61 andthe first-1 signal line path 51 are disposed each in a straight line andobliquely to the longitudinal direction of the first substrate 3 matchedwith the interval between the input end 63 of the second_1 signal linepath 61 and the input end 64 of the second_2 signal line path 62.

The interval between the input end 63 of the second_1 signal line path61 and the input end 64 of the second_2 signal line path 62 may belonger than the interval between the input end 53 of the first-1 signalline path 51 and the input end 54 of the first-2 signal line path 52 asin the optical modulator 1 depicted in FIG. 3. The interval between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 may be longer than the interval betweenthe input end 31 of the first signal electrode 14 and the input end 32of the second signal electrode 24 as in the optical modulator 1 depictedin FIG. 4. Other components in the fifth example are identical to thoseof the optical modulator 1 depicted in FIG. 1 and therefore, are giventhe same reference numerals used in FIG. 1 and will not again bedescribed.

According to the optical modulator 1 depicted in FIG. 6, the length canbe reduced of the signal path from the input end 64 of the second_2signal line path 62 to the output end 56 of the first-2 signal line path52. Therefore, the band of the second optical modulating unit 7 can bewidened. The length of the signal path from the input end 64 of thesecond_2 signal line path 62 to the output end 56 of the first-2 signalline path 52, and the length of the signal path from the input end 63 ofthe second_1 signal line path 61 to the output end 55 of the first-1signal line path 51 can both be reduced maintaining the balance betweenthe lengths of these signal paths. Therefore, the propagation loss ofany high-frequency signal can be reduced and the modulation band can bewidened. The driving voltage during high-speed driving can be reduced.

FIG. 7 is an explanatory diagram of a sixth example of the opticalmodulator according to the embodiment. In an optical modulator 1depicted in FIG. 7, the central line C_(f) between the input end 63 ofthe second_1 signal line path 61 and the input end 64 of the second_2signal line path 62 deviates from the central line C_(c) between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 in the optical modulator 1 depicted inFIG. 1.

The second_2 signal line path 62, the first-2 signal line path 52, and aportion of the second signal electrode 24 spanning from the input end 32to the starting point of the portion of the second signal electrode 24located along the third branching waveguide 20 are disposed each in astraight line and oblique to the longitudinal direction of the firstsubstrate 3. The second_1 signal line path 61, the first-1 signal linepath 51, and a portion of the first signal electrode 14 spanning fromthe input end 31 to the starting point of the portion of the firstsignal electrode 14 located along the first branching waveguide 10 maybe disposed each in a straight line and oblique to the longitudinaldirection of the first substrate 3.

The interval between the input end 63 of the second_1 signal line path61 and the input end 64 of the second_2 signal line path 62 may belonger than the interval between the input end 53 of the first-1 signalline path 51 and the input end 54 of the first-2 signal line path 52 asin the optical modulator 1 depicted in FIG. 3. The interval between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 may be longer than the interval betweenthe input end 31 of the first signal electrode 14 and the input end 32of the second signal electrode 24 as in the optical modulator 1 depictedin FIG. 4. Other components in the sixth example are identical to thoseof the optical modulator 1 depicted in FIG. 1 and therefore, are giventhe same reference numerals used in FIG. 1 and will not again bedescribed.

According to the optical modulator 1 depicted in FIG. 7, such a lengthof the signal path can be reduced as that from the input end 64 of thesecond_2 signal line path 62 to the starting point of a portion of thesecond signal electrode 24 located along the third branching waveguide20. Therefore, the band of the second optical modulating unit 7 can bewidened. The length of the signal path from the input end 64 of thesecond_2 signal line path 62 to the starting point of the portion of thesecond signal electrode 24 located along the third branching waveguide20, and the length of the signal path from the input end 63 of thesecond_1 signal line path 61 to the starting point of a portion of thefirst signal electrode 14 located along the first branching waveguide 10can both be reduced maintaining the balance between the lengths of thesesignal paths. Therefore, the propagation loss of any high-frequencysignal can be reduced and the modulation band can be widened. Thedriving voltage during high-speed driving can be reduced.

FIG. 8 is an explanatory diagram of a seventh example of the opticalmodulator according the embodiment. In an optical modulator 1 depictedin FIG. 8, DC electrodes 81 and 82 are further included respectivelydisposed along the first and the second output waveguides 13 and 23 inthe optical modulator 1 depicted in FIG. 1. In the optical modulator 1depicted in FIG. 8, a bias 83 may be supplied to the DC electrodes 81and 82 and, thereby, the phases of the outputs from the two Mach-Zehnderoptical waveguides may be adjusted such that the difference between thephases is 90 degrees. Each of the outputs from the Mach-Zehnder opticalwaveguides may be modulated based on differential phase shift keying(DPSK) that modulates each of the outputs using two values of “0” and“π”. Thereby, a quadrature phase shift keying (QPSK) modulation signalcan be produced.

The second_2 signal line path 62 may be disposed obliquely to thelongitudinal direction of the first substrate 3 as in the opticalmodulator 1 depicted in FIG. 5. The second_1 signal line path 61 may bedisposed obliquely to the longitudinal direction of the first substrate3 matched with the interval between the input end 63 of the second_1signal line path 61 and the input end 64 of the second_2 signal linepath 62.

The interval between the input end 63 of the second_1 signal line path61 and the input end 64 of the second_2 signal line path 62 may belonger than the interval between the input end 53 of the first-1 signalline path 51 and the input end 54 of the first-2 signal line path 52 asin the optical modulator 1 depicted in FIG. 3. The interval between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 may be longer than the interval betweenthe input end 31 of the first signal electrode 14 and the input end 32of the second signal electrode 24 as in the optical modulator 1 depictedin FIG. 4. Other components in the seventh example are identical tothose of the optical modulator 1 depicted in FIG. 1 and therefore, aregiven the same reference numerals used in FIG. 1 and will not again bedescribed.

Similarly, the optical modulator 1 depicted in FIGS. 3 and 4 to 7 may beadapted to produce a QPSK modulation signal.

FIG. 9 is an explanatory diagram of an eighth example of the opticalmodulator according to the embodiment. In an optical modulator 1depicted in FIG. 9, the output from the one Mach-Zehnder opticalwaveguide whose polarization beam is rotated by 90 degrees by apolarization beam rotating device 91 is coupled with the output from theother Mach-Zehnder optical waveguide by a polarization beam combiner(PBC) 92 in the optical modulating apparatus 1 depicted in FIG. 1.Thereby, a polarization beam multiplexed signal can be produced andoutput from the polarization beam combiner 92.

The second_2 signal line path 62 may be disposed obliquely to thelongitudinal direction of the first substrate 3 as in the opticalmodulator 1 depicted in FIG. 5. The second_1 signal line path 61 may bedisposed obliquely to the longitudinal direction of the first substrate3 matched with the interval between the input end 63 of the second_1signal line path 61 and the input end 64 of the second_2 signal linepath 62.

The interval between the input end 63 of the second_1 signal line path61 and the input end 64 of the second_2 signal line path 62 may belonger than the interval between the input end 53 of the first-1 signalline path 51 and the input end 54 of the first-2 signal line path 52 asin the optical modulator 1 depicted in FIG. 3. The interval between theinput end 53 of the first-1 signal line path 51 and the input end 54 ofthe first-2 signal line path 52 may be longer than the interval betweenthe input end 31 of the first signal electrode 14 and the input end 32of the second signal electrode 24 as in the optical modulator 1 depictedin FIG. 4. Other components in the eighth example are identical to thoseof the optical modulator 1 depicted in FIG. 1 and therefore, are giventhe same reference numerals used in FIG. 1 and will not again bedescribed.

Similarly, the optical modulator 1 depicted in each of FIGS. 3 and 4 to7 may be adapted to produce a polarization beam multiplexed signal.

FIG. 10 is an explanatory diagram of an example of an opticaltransmitter according to the embodiment. As depicted in FIG. 10, theoptical transmitter 101 includes an optical modulator 102, a lightemitting element 103, a data generating circuit 104, and a driver 105.

The light emitting element 103 emits a light beam. A laser diode (LD) isan example of the light emitting element 103. The data generatingcircuit 104 produces modulation data. The driver 105 produces anelectrical signal having an amplitude that corresponds to the modulationdata output from the data generating circuit 104. The optical modulatingunit 102 executes the modulation of the light beam emitted from thelight emitting element 103, based on the electrical signal output fromthe driver 105. The optical modulator 1 depicted in each of the FIGS. 1,3 and 4 to 9 is an example of the optical modulator 102 of the opticaltransmitter 101. The light beam output from the optical modulator 102may be output to an optical fiber 106 through a connector not depicted.

According to the optical transmitter depicted in FIG. 10, use of theoptical modulator 1 depicted in each of FIGS. 1, 3, and 4 to 9 as theoptical modulator 102 enables reduction of the size of a package of theoptical modulator 102. Even when the pitch of the plural signal wiresconnecting the driver 105 and the optical modulator 102 to each other iswider than the pitch of the signal electrodes of the modulator chip,conversion of the pitch can be executed on the flexible circuit board ofthe optical modulator 102.

The optical modulator 1 depicted in each of FIGS. 1, 3, and 4 to 9includes the two Mach-Zehnder optical waveguides and the two signalelectrodes. However, the above is similarly applied to the case wherethe optical modulator 1 includes three or more Mach-Zehnder opticalwaveguides and three or more signal electrodes.

According to the optical modulator and the optical transmitter, the sizeof the package of the optical modulator can be reduced.

All examples and conditional language provided herein are intended forpedagogical purposes of aiding the reader in understanding the inventionand the concepts contributed by the inventor to further the art, and arenot to be construed as limitations to such specifically recited examplesand conditions, nor does the organization of such examples in thespecification relate to a showing of the superiority and inferiority ofthe invention. Although one or more embodiments of the present inventionhave been described in detail, it should be understood that the variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. An optical modulator comprising: a firstsubstrate that includes: a Mach-Zehnder optical waveguide that is formedon the first substrate having an electro-optical effect; a signalelectrode and a ground electrode that are disposed along a pair ofbranching waveguides positioned between an optical branching unit and anoptical coupling unit of the Mach-Zehnder optical waveguide; pluraloptical modulating units that are disposed in parallel, and respectivelymodulate a light beam propagating in the Mach-Zehnder optical waveguideby applying to the signal electrode that is to function as a travellingwave electrode, an electrical signal that corresponds to modulationdata; and an output optical coupling unit that couples modulated lightbeams output from the optical modulating units; a second substrate thatis disposed separately from the first substrate and has plural firstsignal line paths corresponding to the optical modulating units; apackage that accommodates therein the first substrate and the secondsubstrate; and a flexible circuit board that is disposed outside thepackage and has plural second signal line paths corresponding to theoptical modulating units, wherein input ends of input electrodes of theoptical modulating units on the first substrate are each supplied theelectrical signal and are disposed side by side on one side face of thefirst substrate, when base points are independently set such that eachoptical path length from the output optical coupling unit in thebranching waveguide disposed along the signal electrode of the branchingwaveguides of the optical modulating units to the base points isequivalent, each electrical length from an input end of each signalelectrode to each respective base point differs, input ends of the firstsignal line paths on the second substrate are respectively supplied theelectrical signals respectively corresponding to the optical modulatingunits and are disposed side by side on one side face of the secondsubstrate, and output ends electrically connected to input ends of thesignal electrodes of the corresponding optical modulating units on thefirst substrate are disposed side by side on another side face of thesecond substrate, input ends of the second signal line paths on theflexible circuit board are respectively supplied the electrical signalsrespectively corresponding to the optical modulating units and aredisposed side by side on one side face of the flexible circuit board,and output ends electrically connected to input ends of thecorresponding first signal line paths on the second substrate aredisposed side by side on another side face of the flexible circuitboard, electrical lengths from the input ends of the second signal linepaths to output ends thereof respectively differ, and electrical lengthsof signal paths respectively corresponding to the optical modulatingunits are equivalent, the signal paths include signal paths that arefrom the input ends of the second signal line paths and pass throughinput ends of the first signal line paths connected to the output endsof the second signal line paths, to the base points on the signalelectrodes, and signal paths that are from the input ends of the firstsignal line paths and pass through the input ends of the signalelectrodes connected to output ends of the first signal line paths, tothe base points on the signal electrodes.
 2. The optical modulatoraccording to claim 1, wherein an interval between the input ends of thesecond signal line paths on the flexible circuit board is different froman interval that corresponds thereto and is between the input ends ofthe first signal line paths on the second substrate.
 3. The opticalmodulator according to claim 2, wherein the interval between the inputends of the second signal line paths on the flexible circuit board islonger than the interval that corresponds thereto and is between theinput ends of the first signal line paths on the second substrate, andthe interval between input ends of the first signal line paths on thesecond substrate is longer than an interval that corresponds thereto andis between input ends of the signal electrodes on the first substrate.4. The optical modulator according to claim 1, wherein a central linebetween the adjacent input ends of the second signal line paths on theflexible circuit board deviates from a central line that correspondsthereto and is between the adjacent input ends of the first signal linepaths on the second substrate.
 5. The optical modulator according toclaim 1, wherein the second signal line paths on the flexible circuitboard are disposed obliquely to a longitudinal direction of the firstsubstrate.
 6. The optical modulator according to claim 5, wherein thefirst signal line paths on the second substrate are disposed obliquelyto the longitudinal direction of the first substrate, and the firstsignal line paths and the corresponding second signal line paths on theflexible circuit board are each disposed in a straight line.
 7. Theoptical modulator according to claim 6, wherein a portion spanning froman input end of each of the signal electrodes on the first substrate toa portion located along the branching waveguide is disposed obliquely tothe longitudinal direction of the first substrate, and a portionspanning from an input end of each of the signal electrodes to a portionlocated along the branching waveguide, the corresponding first signalline path on the second substrate, and the corresponding second signalline path on the flexible circuit board are respectively formed in astraight line.
 8. The optical modulator according to claim 1, whereinsums corresponding to each of the optical modulating units areequivalent, the sums respectively being a sum of a product of a lengthand a loss per unit length of the second signal line paths on theflexible circuit board, a product of a length and a loss per unit lengthof the first signal line paths on the second substrate, and a product ofa loss per unit length of the signal electrodes on the first substrateand a length of a signal path from the input end of the signal electrodeto the base point.
 9. The optical modulator according to claim 1,wherein the optical modulator is a QPSK modulator.
 10. The opticalmodulator according to claim 1, wherein the optical modulator is apolarization beam multiplex modulator.
 11. An optical transmittercomprising: a light emitting element that emits light; a data generatingcircuit that generates modulation data; a driver that generates anelectrical signal corresponding to the modulation data output from thedata generating circuit; and an optical modulator that based on theelectrical signal output from the driver, modulates the light emittedfrom the light emitting element; wherein the optical modulator includes:a first substrate that includes: a Mach-Zehnder optical waveguide thatis formed on the first substrate having an electro-optical effect; asignal electrode and a ground electrode that are disposed along a pairof branching waveguides positioned between an optical branching unit andan optical coupling unit of the Mach-Zehnder optical waveguide; pluraloptical modulating units that are disposed in parallel, and respectivelymodulate a light beam propagating in the Mach-Zehnder optical waveguideby applying to the signal electrode that is to function as a travellingwave electrode, an electrical signal that corresponds to modulationdata; and an output optical coupling unit that couples modulated lightbeams output from the optical modulating units; a second substrate thatis disposed separately from the first substrate and has plural firstsignal line paths corresponding to the optical modulating units; apackage that accommodates therein the first substrate and the secondsubstrate; and a flexible circuit board that is disposed outside thepackage and has plural second signal line paths corresponding to theoptical modulating units, wherein input ends of input electrodes of theoptical modulating units on the first substrate are each supplied theelectrical signal and are disposed side by side on one side face of thefirst substrate, when base points are independently set such that eachoptical path length from the output optical coupling unit in thebranching waveguide disposed along the signal electrode of the branchingwaveguides of the optical modulating units to the base points isequivalent, each electrical length from an input end of each signalelectrode to each respective base point differs, input ends of the firstsignal line paths on the second substrate are respectively supplied theelectrical signals respectively corresponding to the optical modulatingunits and are disposed side by side on one side face of the secondsubstrate, and output ends electrically connected to input ends of thesignal electrodes of the corresponding optical modulating units on thefirst substrate are disposed side by side on another side face of thesecond substrate, input ends of the second signal line paths on theflexible circuit board are respectively supplied the electrical signalsrespectively corresponding to the optical modulating units and aredisposed side by side on one side face of the flexible circuit board,and output ends electrically connected to input ends of thecorresponding first signal line paths on the second substrate aredisposed side by side on another side face of the flexible circuitboard, electrical lengths from the input ends of the second signal linepaths to output ends thereof respectively differ, and electrical lengthsof signal paths respectively corresponding to the optical modulatingunits are equivalent, the signal paths include signal paths that arefrom the input ends of the second signal line paths and pass throughinput ends of the first signal line paths connected to the output endsof the second signal line paths, to the base points on the signalelectrodes, and signal paths that are from the input ends of the firstsignal line paths and pass through the input ends of the signalelectrodes connected to output ends of the first signal line paths, tothe base points on the signal electrodes.
 12. The optical transmitteraccording to claim 11, wherein an interval between the input ends of thesecond signal line paths on the flexible circuit board is longer thanthe interval that corresponds thereto and is between the input ends ofthe first signal line paths on the second substrate, an interval betweeninput ends of the first signal line paths on the second substrate islonger than an interval that corresponds thereto and is between inputends of the signal electrodes on the first substrate, a central linebetween the adjacent input ends of the second signal line paths on theflexible circuit board deviates from a central line that correspondsthereto and is between the adjacent input ends of the signal electrodeson the first substrate.
 13. The optical transmitter according to claim11, wherein the second signal line paths on the flexible circuit boardare disposed obliquely to a longitudinal direction of the firstsubstrate, the first signal line paths on the second substrate aredisposed obliquely to the longitudinal direction of the first substrate,and the first signal line paths and the corresponding second signal linepaths are each disposed in a straight line.
 14. The optical transmitteraccording to claim 11, wherein sums corresponding to each of the opticalmodulating units are equivalent, the sums respectively being a sum of aproduct of a length and a loss per unit length of the second signal linepaths on the flexible circuit board, a product of a length and a lossper unit length of the first signal line paths on the second substrate,and a product of a length and a loss per unit length of the signalelectrodes on the first substrate.